The purpose of this study is to elucidate the mechanisms with which immunoglobulins interact with effectors of the immune system subsequent to complexing with antigens. Models for immune complexes have been prepared by covalently cross-linking antibody molecules using two different bivalent affinity labels and dimethylsuberimidate. Stable oligomers of defined size have been isolated. Radioactive oligomers have been used to investigate the immunoglobulin binding properties of cells via Fc receptors. Several types of human cells have been isolated; although the number of receptors for IgG varies between cell populations, the affinities of each cell type for oligomeric IgG are the same for each cell within experimental error. Oligomers have been attached to TNP-modified sheep erythrocytes and the size requirements for eliciting complement-mediated lysis investigated. The results suggest that the ability of IgG to initiate lysis increases gradually with size and that there is no defined signal for eliciting lysis. In contrast, cell mediated lysis of oligomer-coated cells is independent of cluster size and depends only upon the number of IgG molecules bound per cell.